The increasing demand for very-large-scale-integrated (VLSI) devices has made imperative the development of techniques for multilevel metallization, multilayer resist processing, and the like. These developments necessitate utilization of layers of planarizing material to smooth uneven topography on the substrate so that there will be minimal thickness variation in, e.g., a subsequently spun-on coating of resist material. The need to effectively planarize such resist coatings increases as device dimensions shrink.
In addition to causing changes in the thickness of overlying layers, substrate topography can cause optical interference by reflection of the irradiating beam. This can be very detrimental to precise pattern replication in the resist layer. The problem is often exacerbated by the irradiation source itself. The fine-line resolution required for pattern definition approaching one micrometer is conventionally obtained with step-and-repeat projection imaging equipment utilizing essentially monochromatic light. Monochromatic light, however, generates a high incidence of reflection interference. Furthermore, the depth-of-focus of lenses for projection imaging equipment is being reduced in an effort to obtain submicrometer resolution. The use of such lenses increases the need for the surface of the photoresist structure to be as even as possible. These problems are conventionally alleviated by using a multilayer resist system.
A multilayer resist system is comprised of a thick planarizing layer and a thin photoresist imaging layer which may be separated by a thin inorganic hardmask layer impervious to oxygen reactive etching (RIE). The imaging layer is irradiated, developed and the exposed portion of the underlying planarizing and, if present, hardmask layers removed. The system containing the hardmask layer is generally regarded as providing better resolution. It is conventional with either system to incorporate a dye into the planarizing layer to prevent loss of resolution resulting from substrate reflection.
The selection of a suitable dye for multilayer photoresist structures is complicated by the fact that a number of criteria must be met. An acceptable dye must be exceptionally pure to prevent contamination of the device being fabricated, have a high solubility in the high-boiling solvent utilized to spin-coat the substrate and have the proper absorbance characteristics. This latter requirement is the most demanding because of improvements in irradiation equipment. The newer imaging apparatus, termed "I-line", utilize wavelengths below 436 nm, e.g. 366 nm, to irradiate the photoresist layer to maximize resolution, and longer wavelengths, i.e. 514 nm and above, to align the exposures, i.e. the imaging mask with the substrate. To insure proper alignment, a dye must transmit light at the higher wavelengths while effectively absorbing light at the irradiating wavelengths.
In addition to the problem of dye selection, there also remains the problem that, even though multilayer systems provide improved resolution over a single layer of photoresist, it is difficult to effectively planarize large topographical features with the planarizing materials conventionally in use at the present time. Examples of large topographical features include island features in excess of about 30 micrometers square, particularly those in excess of about 50 micrometers square, and other features of comparable dimensions. It is necessary to effectively planarize such large features in order to obtain high resolution from the photoresist pattern. The difficulty in achieving effective planarization will be appreciated when it is considered that, using a thin imaging photoresist layer, i.e. about 0.5 micrometer in thickness, a step height greater than about 0.1 micrometer in the planarizing layer will produce greater than 20 percent deviation in the thickness of the imaging resist layer.
In accordance with this invention there is provided a material with excellent planarizing properties which additionally possesses the requisite absorbing properties discussed above without the incorporation of a dye.